the search for the shortest period binary wds: the latest...
TRANSCRIPT
The ELM Survey: A Targeted Search for the Most
Extreme White Dwarf Binary Systems
Alex Gianninas Astronomy Colloquium
Wesleyan University February 25th, 2015
Outline
• Intro: Stellar Evolution & White Dwarfs 101
• Extremely Low Mass White Dwarfs
• The ELM Survey
• Highlights from 2014
• Takeaways
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Stellar Evolution and White Dwarfs 101
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• No H burning, WDs cool over several billion years (Gyrs)
• Teff ~ 150,000 K → 3000 K • M ~ 0.6 M • R ~ 0.01 R ~ 1 R⊕ • log g ~ 8.0 (log g = 4.4, log g ⊕ = 3.0)
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Stellar Evolution and White Dwarfs 101
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• No H burning, WDs cool over several billion years (Gyrs)
• Teff ~ 150,000 K → 3000 K • M ~ 0.6 M • R ~ 0.01 R ~ 1 R⊕ • log g ~ 8.0 (log g = 4.4, log g ⊕ = 3.0)
𝒈𝒈 = 𝑮𝑮𝑮𝑮𝑹𝑹𝟐𝟐
Gravitational settling quickly creates a stratified internal structure
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C/O
He H
DA (H-rich)
C/O
He
Non-DA (He-rich)
C/O
Hot DQ
Extremely Low Mass (ELM) WDs have M < 0.3 M and form in compact binaries
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Gianninas et al. (2011, ApJ, 743, 138)
Binary Evolution
Extremely Low-Mass (ELM) WDs have He cores and are formed in compact binaries
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Iben & Tutukov (1984, ApJ, 54, 335)
ELM WDs have He cores and considerably larger radii
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C/O
He H
He
H
ELM
𝑹𝑹~𝟏𝟏𝑮𝑮𝟑𝟑
The ELM Survey • The ELM Survey is an ongoing, targeted search
for ELM WDs – M < 0.30 M (5 < log g < 7) – Found in short-period (P < 1 day) binary systems
• Motivation – Progenitors of SN Ia, .Ia, AM CVn, R CrB – Neutron star companions – Gravitational wave verification sources – Laboratories to test GR, tidal effects
• Papers I – V + VI : 62 + 12 new ELM WDs
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SDSS colors work well for choosing ELM WD candidates
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Brown et al. (2012, ApJ, 744, 142) Gianninas et al. (2015, in prep)
Radial Velocity Follow-Up
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FLWO 1.5m MMT 6.5m KPNO 4m
The optical spectra of ELM WDs are dominated by the Hydrogen Balmer lines
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More Balmer lines are visible in ELM WDs due to the lower log g
DA ELM
Hβ Hγ Hγ Hδ
ELM VI: 12 new ELM WDs
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Gianninas et al. (2015, in prep) 𝑷𝑷𝑲𝑲𝟑𝟑
𝟐𝟐𝝅𝝅𝑮𝑮= 𝑮𝑮𝒇𝒇 =
(𝒎𝒎𝟐𝟐𝐬𝐬𝐬𝐬𝐬𝐬 𝒊𝒊)𝟑𝟑 (𝒎𝒎𝟏𝟏+𝒎𝒎𝟐𝟐)𝟐𝟐
Hydrogen Balmer lines are very sensitive to both Teff and log g
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Standard Spectroscopic Technique
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Gianninas et al. (2015, in prep)
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Gianninas et al. (2015 ApJ, in prep)
There is still a lot of uncertainty in the evolutionary models
• Althaus et al. (2013, A&A, 557, A19)
• Istrate et al. (2014, A&A, 571, L3)
Some ELM WDs may be Type Ia progenitors
• Super-Chandrasekhar mass systems not necessarily Type Ia progenitors
• WD+NS binaries? No radio or X-ray detections
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Brown et al. (2013, ApJ, 769, 66)
Some ELM WDs are likely progenitors of AM CVn systems
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Gianninas et al. (2015, in prep)
Cooler ELM WD binaries have longer periods or else they would have already merged
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Gianninas et al. (2015, in prep)
K-S statistic = 0.5622
INDIVIDUAL HIGHLIGHTS
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J0651: the poster child for ELM WDs
• Shortest period ELM WD binary → P = 12.75 min!
• Eclipsing!
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Hermes et al. (2012, ApJ, 757, 21)
The shift in the midpoint of the eclipses reveals that the orbit is decaying !
23 Hermes et al. (2012, ApJ, 757, 21)
The rate of decay agrees with the prediction of General Relativity!
24 Hermes et al. (2015, in prep)
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PSR B1913+16: The Hulse-Taylor pulsar
Weisberg & Taylor (2005, ASPC, 328, 25)
• Orbital decay of a binary pulsar system (PSR +NS)
• Test of GR • 1993 Nobel Prize in
Physics • It took 30 years to
display the same period shift as J0651!
WD 0931+444: a new 20-min ELM WD!
• Preliminary SDSS DR10 selection
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Kilic et al. (2014, MNRAS, 441, L1 )
WD 0931+444 looks like a DA+dM…
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Kilic et al. (2014, MNRAS, 441, L1 )
MMT 6.5m SDSS
DA+dM1 : Silvestri et al. (2006) Rebassa-Mansergas et al. (2007,2010) Heller et al. (2009)
Gemini data shows that M dwarf is indeed a background object
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Kilic et al. (2014, MNRAS, 441, L1 )
GMOS-North time resolved spectroscopy
Na I doublet Hα
ELM WDs are among the best gravitational wave verification sources for eLISA
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Kilic et al. (2015, ASSP, 40, 167)
Average Galactic Foreground Nelemans et al. (2001)
eLISA after 2 years Amaro-Seoane et al. (2013)
Several PSR have ELM WD companions
• Recent example: triple system PSR J0337+1715 (Ransom et al., 2014, Nature)
• Several of the ELM WD companions are in the correct range of Teff and log g (i.e. instability strip) to pulsate
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ELM WD instability strip is an extension of ZZ Ceti Instability Strip
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Gianninas et al. (2014, ApJ, 794,35)
Several PSR have ELM WD companions
• Recent example: triple system PSR J0337+1715 (Ransom et al., 2014, Nature)
• Several of the ELM WD companions are in the correct range of Teff and log g (i.e. instability strip) to pulsate
• No pulsations detected for ELM companions of PSR J1012+5307+ and PSR J1911-5958 (Steinfadt et al. 2010)
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PSR J1909-3744
• No strong evidence for pulsations
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Kilic et al. (2015, MNRAS, 446,L26)
PSR J1738+0333
• First pulsating ELM WD companion to a PSR!
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Kilic et al. (2015, MNRAS, 446, L26)
Updated ELM WD instability strip
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Kilic et al. (2015, MNRAS, 446, L26)
TAKEAWAYS
• ELM Survey has discovered 74 ELM WDs and many more are in the pipeline – Starting to be able to do some statistics
• 2014 Highlights: – J0651 : detection of orbital decay
– Discovery of a new 20-min system : 2nd gravitational wave verification source
– Discovery of first PSR + pulsating ELM WD
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Collaborators
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University of Oklahoma: Mukremin Kilic, Paul Canton Smithsonian Astrophysical Observatory: Warren Brown, Scott Kenyon University of Warwick: JJ Hermes Université de Montréal: Patrick Dufour, Pierre Bergeron University of Texas: Keaton Bell, Samuel Harrold Instituto de Astrofisica de Canarias: Carlos Allende Prieto